This invention relates to a novel transcription factor and to its production and uses.
The molecular events that occur during the cell cycle need to be integrated with the transcription apparatus so that gene expression can be synchronised with cell cycle progression.
Recently, a transcription factor called E2F (or DRTF1) has been identified and shown to bind to pRb, the protein product of the retinoblastoma susceptibility gene, an anti-oncogene or tumour suppressor gene (see for example Wagner and Green, Nature 352, 189-190, 1991). It is widely believed that the cellular transcription factor E2F functions as a key component in cell cycle control because it associates with important cell cycle regulating proteins, such as the retinoblastoma gene product (pRb), p107, cyclins and cyclin-dependent kinases, and furthermore its transcriptional activity is modulated by certain viral oncoproteins, such as adenovirus Ela, SV40 large T antigen, and the human papilloma virus E7 protein.
It is believed that the transcription factor E2F (or DRTF1) plays an important role in integrating cell cycle events with the transcription apparatus because, during cell cycle progression in mammalian cells, it undergoes a series of periodic interactions with molecules that are known to be important regulators of cellular proliferation. For example, the retinoblastoma tumour suppressor gene product (pRb), which negatively regulates progression from G1 into S phase. and is frequently modified in tumour cells, binds to E2F. Similarly, the pRb-related protein p107 occurs predominantly in an S phase complex with E2F. Both pRb and p107 repress the transcriptional activity of E2F, which is likely to be fundamentally important for regulating cellular proliferation because E2F binding sites occur in the control regions of a variety of genes that are involved with proliferation, such as c-myc and p34cdc2. Furthermore, mutant Rb proteins, encoded by alleles isolated from tumour cells, fail to bind to E2F, and hence are unable to interfere with E2F site-dependent transcriptional activation. Another important feature of E2F is that certain viral oncoproteins, such as adenovirus Ela, SV40 large T antigen and human papilloma virus E7, modulate its activity by sequestering pRb and p107 from the inactive transcription factor. This effect requires regions in these viral proteins that are necessary for transformation of tissue culture cells and hence to overcome growth control. Thus, the ability of these oncoproteins to regulate E2F may be the means by which they over-ride the normal mechanisms of cellular growth control and, conversely, transcriptional repression by pRb may be the basis of pRb-mediated negative growth control.
A potential mechanism for integrating the transcription-regulating properties of pRb and p107 with other cell cycle events was suggested by the identification of cyclin A and the cdc2-related cyclin-dependent kinase p33cdk2 in the E2F complex. Cyclin A is necessary for progression through S phase, a function that could perhaps be mediated through its ability to recruit the cyclin-dependent kinase p33cdk2 to E2F. Taken together these data suggest that E2F is a transcription factor whose primary role may be to relay cell cycle events to the transcription apparatus via molecules such a pRb, p107, cyclins and cdks, thus ensuring that gene expression is synchronised and integrated with cell cycle progression.
More recently, a transcription factor with the properties of E2F has been cloned and sequenced (Helin et al, Cell 70 (1992), 337-350 and Kaelin et al, Cell 70 (1992), 351-364).
We have now surprisingly found a further two new proteins which appear to be new members of the E2F gene family, which we have called E2F-5. The cDNA sequence of human E2F-5 is presented in FIG. 1A, as is the amino acid sequence of this protein. The corresponding sequences for murine E2F-5 appear in FIG. 9A. These new proteins are referred to as E2F-5 and this nomenclature will be used in this specification.